The ability to access electricity is a crucial factor in both the overall economic growth of a country and the overall quality of life of its inhabitants. In most parts of the world, areas without electricity are far less developed than areas with electricity. In areas with limited conventional energy reserves, rising demand for power coupled with inadequate power generation has made providing reliable electrical power without frequent disruption a major challenge.
In order to address the issue of providing reliable power and to tackle the challenge of climate change, there is an increasing focus on enhancing the use of renewable energy sources. The high levels of solar insolation in some developing countries, and inability to meet the demand for electricity through conventional energy sources, has encouraged local governments to promote schemes such as solar street lighting and solar rooftop photovoltaic (PV) systems by providing incentives and attractive power grid feed-in tariffs.
This has made PV systems increasingly popular. In order to make effective use of such government initiatives it is important to look for ways to efficiently utilize the power generated in solar PV systems with grid feed-in capability so as to not only reduce the capital cost, granting more accessibility to a wider range of users, but to also improve the grid reliability in weak grid regions.
Document WO2015/169131 describes the distribution of energy from the PV array to the battery and to the power grid at the same time. However, a simultaneous distribution such as this requires two control loops for the battery and the grid to be integrated together in order to obtain satisfactory performance from the system. This is a highly complex process.
A time-divided power distribution between the battery and the grid would be more suitable for simple use. A time-divided power distribution means the power is routed to either the battery or the grid at a given time, instead of being routed to both.
For providing safety and convenience to road users, PV based street lighting systems are becoming more popular as they are independent of the grid and so illuminate roads regardless of power grid conditions. For PV based street lighting systems, it is found that better utilization of the generated energy can be made by opting for a centralized distribution architecture over de-centralized systems. Various such centralized distribution architectures were investigated for their efficiency and complexity in street lighting applications and a narrow bus DC-centralized distribution architecture was found to render superior efficiency over other distribution architectures. DC-distribution architectures allow the use of DC based linear LED drivers, resulting in further energy savings over AC-distribution architectures.
The considerable fall in PV system prices over recent years and awareness of the negative environmental impact of batteries has resulted in new techniques for optimal sizing of PV systems for street lighting. These optimal PV system designs take the insolation levels on days with bad weather conditions into account, resulting in a system having high PV power generation capability and minimum battery charging requirements.
Such a system generates enough energy on cloudy days, with minimum insolation, to satisfy the energy requirement of the connected load. On days with a clear sky, optimally sized systems produce surplus energy which can be sold to utility companies to bring better return on the capital cost as well as bridge the gap between demand and supply of power in weak grid areas.
Most of the PV systems used for street lighting with grid feed-in capability use simple maximum power point tracking (MPPT) charge controllers and grid feed-in inverters. They are usually discrete and the control strategy employed in such systems allows the energy to be fed to the grid only after the battery is fully charged. For optimally sized systems with large PV power generation capability and low battery charging requirements, such a control strategy provides satisfactory performance on days with low solar insolation; however, on days with good solar insolation, the PV system remains largely underutilized.
This is because, with the increase in solar insolation, the maximum power available at the output of the PV array increases. However, the power required for charging the battery is limited by the battery voltage and charging current requirements. In other words, the power is limited by the maximum input power of the battery. During days of high solar insolation periods, the maximum power available at the PV array output is higher than the battery power requirement. In this situation, the MPPT charge controller is forced to operate the PV array at a lower operating power point equal to the maximum input power of the battery meaning that the excess power, that the PV array could generate, remains unutilized and is wasted.
US20120235497A1 discloses a method of controlling a battery storing electric power generated by a power generator, wherein it supplies to an electric power transmission system electric power corresponding to a target output value from at least one of the power generator and the battery.